docs: def url(self, filename, external=False):
          '''
          This function gets the URL a file uploaded to this set would be
          accessed at. It doesn't check whether said file exists.
  
          :param string filename: The filename to return the URL for.
          :param bool external: If True, returns an absolute URL
          '''
          if filename.startswith('/'):
              filename = filename[1:]
          if self.has_url:
              return self.base_url + filename
          else:
              return url_for('fs.get_file', fs=self.name, filename=filename, _external=external)